The present invention concerns an in quadrature double frequency up-conversion modulator, used in particular in transmitter circuits for mobile telephones. The double up-conversion modulator includes a first frequency up-conversion stage formed by a first modulation unit. This first modulation unit receives at one input baseband quadriphased signals, formed of a first in-phase signal, a second in phase opposition signal, a third in quadrature signal and a fourth in quadrature-opposition signal. This first unit is controlled by carrier signals quadriphased at a first intermediate frequency formed by a first in-phase carrier signal, a second in phase opposition carrier signal, a third in quadrature carrier signal and a fourth in quadrature-opposition carrier signal. The modulation unit delivers a signal up-converted to the first intermediate frequency at one of its ouptut.
This type of simple up-conversion in quadrature modulator, formed of a single modulation unit, is widely known in the prior art. The modulator, such as that shown in FIG. 1, includes two differential mixers 10 and 12. The first mixer 10 receives at one input an in-phase baseband signal bbI and the second mixer 12 receives at one input an in quadrature baseband signal bbQ. The two mixers 10 and 12 are controlled by high frequency carrier signals HF_I and HF_Q obtained by a high frequency in quadrature signal generator, which is not shown. The first mixer 10 is controlled by an in-phase high frequency carrier signal HF_I. The second mixer 12 is controlled by an in quadrature high frequency carrier signal HF_Q, namely a signal phase shifted by 90xc2x0 with respect to carrier signal HF_I. The signals delivered at the outputs of mixers 10 and 12 are in quadrature signals modulated at the frequency of the carrier signals. An adder 14 placed at the output of the modulator adds the two modulated signals to provide the high frequency modulated signal HF_M at its output.
The drawback of such a simple up-conversion modulator lies in the necessity of generating high frequency carrier signals HF_I and HF_Q that are perfectly in quadrature. Such a signal generator quickly becomes complex without however being able to assure a phase shift of 90xc2x0 with sufficient precision. Moreover, the power consumption of such a generator is quite significant.
Another type of simple up-conversion in quadrature modulator known in the prior art is shown in FIG. 2. This modulator operates on exactly the same principle as that presented hereinbefore. The difference from the previous modulator lies in the generation of high frequency in quadrature carrier signals HF_I and HF_Q. Here, a high frequency generator 26 is used to generate carrier signals at a frequency 2HF double the frequency desired for the modulated output signal. An in quadrature frequency divider-by-two 28 is placed at one output of generator 26, which enables carrier signals HF_I and HF_Q to be obtained perfectly in quadrature with the modulation frequency HF desired for controlling differential mixers 20 and 22 of the modulator.
The drawback of this type of modulator lies in the excessive power consumption of high frequency generator 26, used to generate signals at a frequency that is double the desired frequency. Indeed, for frequencies of the order of one GHz, such as for example 900 MHz (mobile telephone range), the generator has to generate signals at a frequency of 1.8 GHz.
One of the permanent concerns of those skilled in the art is making low power consuming circuits, which are generally integrated in portable communication tools, while still obtaining optimum in quadrature signal modulation quality.
The invention is characterised in that it allows the aforementioned drawbacks to be avoided.
The present invention thus concerns a double up-conversion modulator, as defined in the preamble, characterised in that the first stage is also formed of a second modulation unit, similar to the first unit,
receiving at one input said first, second, third and fourth baseband signals,
controlled by said first, second, third and fourth carrier signals at said first intermediate frequency, and
delivering at one output a second converted signal, in phase opposition to said first up-converted signal, at said first intermediate frequency, and in that the modulator further includes a second up-conversion stage,
receiving at one input said first and second up-converted signals at said first intermediate frequency,
controlled by two other carrier signals at a second intermediate frequency, formed by a fifth in-phase carrier signal and a sixth in phase opposition carrier signal, and
delivering at one output a high frequency signal, said high frequency corresponding to the sum of said first and second intermediate frequencies.
The modulator according to the invention thus enables low power consuming modulation to be achieved. Indeed, the carrier signals used to transpose the input baseband signals into a high frequency output signal, are signals at intermediate frequencies lower than the high output modulation frequency. Thus, the lower the frequency, the lower the amount of power consumed by the generator, which is used to provide these carrier signals.
Another constant concern of those skilled in the art is the necessity for generating the cleanest possible modulated signals. In order to do this, it is indispensable to remove all the stray signals or feedthrough signals which appear in unbalanced structures.
This is why, according to an advantageous embodiment of the invention, the double up-conversion modulator is characterised in that the two modulation units of the first up-conversion stage include respectively, first and second differential mixers, and third and fourth differential mixers,
in that the first and second mixers provide at their output, respectively, first and second modulated signals at the first intermediate frequency (IF1), the first and second modulated signals being added at the output of the first unit to obtain the first up-converted signal,
in that the third and fourth differential mixers provide at their output, respectively, third and fourth signals modulated to the first intermediate frequency, the third and fourth modulated signals being added at the output of the second unit to obtain the second up-converted signal, and
in that the second up-conversion stage is formed of a fifth differential mixer.
Finally, in this type of in quadrature modulator, it is very important that the carrier signals are in quadrature and with great precision. Indeed, particularly in a field such as mobile telephony where the modulation frequencies are of the order of one GHz, any phase shift between the in-phase signals and the in quadrature signals leads to the appearance of additional stray signals.
In a preferred embodiment of the invention, the double up-conversion modulator is characterised in that the carrier signals at the second intermediate frequency controlling the second up-conversion stage are directly generated by a signal generator at the second intermediate frequency and in that the carrier signals at the first intermediate frequency controlling the first up-conversion stage are obtained after dividing the frequency by two in an in quadrature frequency divider placed at one output of the signal generator, the second intermediate frequency being twice as high as the first intermediate frequency.